(a) Field of the Invention
The present invention relates to a self-waveguide optical circuit, more in detail to the self-waveguide optical circuit which may easily implement miniaturization, high integration and high speed operation, has an excellent transmission efficiency, and is suitably employed in technical fields such as optical communication, optical control and optical information processing.
(b) Description of the Related Art
Conventional optical circuits can be largely divided into free space propagation optical circuits and waveguide optical circuits.
The free space propagation optical circuit defines optical paths by combining individual optical components such as a collimator, a lens and a reflection mirror without formation of waveguides.
As the collimator for obtaining a bundle of parallel rays among the above optical components, a lens or a concave mirror is employed. A slit or a pinhole formed on the focus of the lens or the concave mirror is precisely adjusted to obtain the bundle of parallel rays.
The waveguide optical circuit is generally realized by forming waveguides by embedding materials having different refractive indexes in the form of railroad tracks and by guiding rays by employing total reflection conditions for rays from the materials.
An example of this waveguide optical circuit is shown in page 272 of volume 4 of advance manuscripts of springtime meeting of Denshi Jouhou Tsuushin Gakkai (Electronics Information and Communication Society) published in 1992 by Takahashi, et al. A top plan view of an array-type waveguide lattice described therein is shown in FIG. 1.
In this example, quartz-based optical waveguides are formed on a waveguide substrate 51 made of silicon, and a waveguide optical circuit includes 11 input ray waveguides 52, an input side star coupler 53, an array-type waveguide lattice 54, an output side star coupler 55 and output waveguides 56.
In another example of the waveguide optical circuit, a possibility is reported that a linear defective structure may be intentionally introduced in a photonic crystal and rays are guided or crooked along the linear defects.
FIG. 2 is a top plan view of a waveguide optical circuit employing photonic crystals described in page 3787, vol.28 of Physical Review Letters in 1996 by A. Mekis et al.
A principle of guiding waves is as follows. The linear defects introduce a guided mode to a wavelength band which has no optical propagation when the crystal is complete. Incident rays selectively exciting this mode are guided along the linear defects. In the ray-propagation mode, only the guided mode is present in the linear defects even in the crooked part. Since, accordingly, incident rays from the incident side-waveguide to the crooked part have no outlet other than the outlet side- waveguides, the rays can propagate along the steep crooked part without loss.
A self-focusing phenomenon may be utilized for self-propagating guided waves without employing structural waveguides. This phenomenon is described, for example, in page 6 of vol.XII of "Progress in Optics" edited by E. Wolf.
This phenomenon can be simply described as follows. A refractive index of a ray in a medium slightly increases with strength of the ray. When, accordingly, a strong bundle of rays passes through the medium, a wavefront is inwardly curved because a phase velocity at the central part thereof having the stronger rays becomes lower than that at the peripheral part thereof having the weaker rays. Although, on the other hand, the bundle of rays is likely to spread due to diffraction, the bundle of rays focuses to one point after propagation of a certain distance if the rays are sufficiently strong and the influence of the refractive index is larger than that of the diffraction.
Accordingly, if the influences of the refractive index and of the diffraction are balanced, the bundle of rays propagates as parallel rays having a specified spread distance. The dependency of the refractive index on the strength of the rays is an effect of a tertiary nonlinear refractive index possessed by a medium known as Kerr effect, and the dependency is proportional to a square of strength of rays.
However, the conventional free space propagation optical circuits and waveguide optical circuits have the following problems.
The free space propagation optical circuits have problems in connection with a collimator. A first problem is that severe conditions are imposed to an accuracy of determining a position of an optical point source or a pinhole, that is, to a divergent angle of an incident ray. This requires a high accuracy of position adjustment of optical elements to elevate a mounting cost. A second problem is that the collimator itself is large and requires a large volume for mounting. This is because a lens as large as an optical source or a pinhole can be regarded as an optical point source is necessary. As a result, a subsidiary problem arises that a spot size of an exit ray becomes larger than the size of the optical source or the pinhole.
In the waveguide optical circuits, waveguides are difficult to be crooked at a steep angle. Since a curvature radius in the order of centimeter is necessary under the present circumstances, the whole circuit inevitably has the order of centimeter and miniaturization is difficult.
Since the total reflection conditions are employed as a principle of guiding rays as mentioned earlier, a steep crook invites a large radiation loss.
Although formation of a complete crystal having a wavelength band in which no propagation of rays occurs and introduction of a linear defective structure are required in the self-waveguide optical circuit employing the photonic crystals, realization of the complete crystal is technically difficult under the present circumstances.
It is extremely difficult to intentionally introduce the defective structure into the complete crystal for forming linear or crooked waveguides. For example, in order to constitute such a linear or crooked waveguide structure in a wavelength band of rays around 1.5 micronmeters employed for current optical communications, control of processing in the order of at least 0.1 micronmeter and a high aspect ratio are required, and these are extremely difficult under the present circumstances of techniques.
Since the effect is proportional to a square of strength of rays in the self-waveguide circuit employing the self-focusing, precise control of the strength of rays is needed for obtaining a balance with a spread of diffraction.
In order to maintain conditions of this balance, steadiness of the strength of rays must be guaranteed. However, absorption is generally increased due to a resonance effect in a wavelength band having a large non-linearity, and a phenomenon in which the strength of rays is attenuated along the propagation direction is difficult to be avoided.
Of course, even a non-resonance region having a lesser absorption effect has a non-linear constant to some degree. In this case the strength of rays must be sufficiently high which reaches to an unrealistic value (watt-level) in an optical circuit having a purpose of information processing or optical connection of a short distance.
As apparent from foregoing, the free space propagation optical circuits requiring the collimator become large-scaled, and the waveguide optical circuits requiring the large curvature radius also become large-scaled. Attainment of miniaturization, high integration and high speed operation is difficult in all of the conventional optical circuits, and accordingly elevation of a transmission efficiency is also difficult.